Add a parser to get font data from the default appearance (#12831)

* Add a parser to get font data from the default appearance
 - pdfium & poppler use a special parser too to get these info.

* Update src/core/default_appearance.js

Co-authored-by: Jonas Jenwald <jonas.jenwald@gmail.com>

Co-authored-by: Jonas Jenwald <jonas.jenwald@gmail.com>

src/core/default_appearance.js

@@ -0,0 +1,96 @@
+/* Copyright 2020 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { isName, Name } from "./primitives.js";
+import { OPS, warn } from "../shared/util.js";
+import { ColorSpace } from "./colorspace.js";
+import { escapePDFName } from "./core_utils.js";
+import { EvaluatorPreprocessor } from "./evaluator.js";
+import { StringStream } from "./stream.js";
+
+class DefaultAppearanceEvaluator extends EvaluatorPreprocessor {
+  constructor(str) {
+    super(new StringStream(str));
+  }
+
+  parse() {
+    const operation = {
+      fn: 0,
+      args: [],
+    };
+    const result = {
+      fontSize: 0,
+      fontName: Name.get(""),
+      fontColor: new Uint8ClampedArray([0, 0, 0]) /* black */,
+    };
+
+    try {
+      while (this.read(operation)) {
+        if (this.stateManager.stateStack.length !== 0) {
+          // Don't get info in save/restore sections.
+          args.length = 0;
+          continue;
+        }
+        const { fn, args } = operation;
+        switch (fn | 0) {
+          case OPS.setFont:
+            const [fontName, fontSize] = args;
+            if (isName(fontName)) {
+              result.fontName = fontName;
+            }
+            if (typeof fontSize === "number" && fontSize > 0) {
+              result.fontSize = fontSize;
+            }
+            break;
+          case OPS.setFillRGBColor:
+            ColorSpace.singletons.rgb.getRgbItem(args, 0, result.fontColor, 0);
+            break;
+          case OPS.setFillGray:
+            ColorSpace.singletons.gray.getRgbItem(args, 0, result.fontColor, 0);
+            break;
+          case OPS.setFillColorSpace:
+            ColorSpace.singletons.cmyk.getRgbItem(args, 0, result.fontColor, 0);
+            break;
+        }
+        args.length = 0;
+      }
+    } catch (reason) {
+      warn(`parseDefaultAppearance - ignoring errors: "${reason}".`);
+    }
+
+    return result;
+  }
+}
+
+// Parse DA to extract font and color information.
+function parseDefaultAppearance(str) {
+  return new DefaultAppearanceEvaluator(str).parse();
+}
+
+// Create default appearance string from some information.
+function createDefaultAppearance({ fontSize, fontName, fontColor }) {
+  let colorCmd;
+  if (fontColor.every(c => c === 0)) {
+    colorCmd = "0 g";
+  } else {
+    colorCmd =
+      Array.from(fontColor)
+        .map(c => (c / 255).toFixed(2))
+        .join(" ") + " rg";
+  }
+  return `/${escapePDFName(fontName.name)} ${fontSize} Tf ${colorCmd}`;
+}
+
+export { createDefaultAppearance, parseDefaultAppearance };